1. Field of the Invention
This invention relates to coatings on glass, more particularly to low reflectance coatings.
2. Background of the Invention
Many different types of devices receive and transmit light through glass. One type of these devices is spatial light modulators. These modulators include area array modulators, as well as opto-electronic or magneto-electronic modulators. Area array modulators typically consist of arrays of individual elements arranged in rows and columns, and are typically used in imaging applications, such as displays and printers. They are illuminated with a source light and modulate that light into an image.
Each individual element on the device typically corresponds to a picture element (pixel) in the produced image. These arrays receive light through the glass and then selected elements transmit light to the display surface. The selection of which transmit light (white) and which do not (black) forms the image. Several sophisticated techniques, such as pulse width modulation for gray scale and field sequential color, can be applied to produce color images. These modulators have several advantages, not the least of which is the ability to control each individual dot on the image.
In order for these arrays to function effectively, stray light in the system must be controlled. Light is constrained to the active area of the array by an opaque coating that prevents source light from striking any part of the device other than the elements. This prevents any reflection from other parts of the device which may cause artifacts in the image. In current embodiments, the glass was assumed to need high reflectance. Light from a source outside the package striking the opaque coating would be reflected. Unfortunately, this high reflectance has been found to cause ghost images around the edge of the image projected by the array. One method of preventing this type of reflectance would require the opaque aperture coating to absorb light.
However, if the opaque coating absorbs too much light, it overheats the package as well. Therefore, a coating is needed that defines the optical aperture with low reflectance from the outside of the package and that does not heat the package beyond its limits.